Your partner gravitated toward friends who are poly because she recognized kindred spirits--people who were practicing a way of life that appeals to her.

They didn't suddenly make her poly, and moving to a new area didn't suddenly make her poly.

She probably always had inclinations toward a polyamorous philosophy of relationships, even if she wasn't aware of how to articulate those feelings.

Probably she couldn't talk to you about it earlier (or even now) because you shut down completely at the thought of even reading a book about poly. Even though reading this book could help you to understand where your partner is coming from.

Your fears that she will leave you are coming from your own insecurities.

Poly people don't leave their partners just because they want to date other people.

That's actually what monogamous people do--if you fall for someone else, you leave the person you are with.